1. Field of the Invention
The invention relates to wireless communication. In particular, the invention relates to frequency tracking and time tracking in a wireless communication system.
2. Description of the Related Art
Communication over a wireless channel can be accomplished using a variety of techniques which facilitate a large number of users in a limited frequency spectrum. These techniques, commonly referred to as multiple access techniques, include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA offers many advantages over other modulation techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In CDMA and other wireless communication technologies, frequency tracking and time tracking loops are often used to monitor and adjust the frequency and timing of received signals. For example, frequency variations, or errors, often exist in carrier signals transmitted between a wireless communication device (WCD) and a base station. Typically, a frequency tracking loop is used to monitor the frequency variations and frequency sensitive components are adjusted accordingly. In addition, frequency variations can adversely affect timing tracking loops used to maintain timing references in the WCD and the base station.
A major contribution to frequency variations in wireless communications is the Doppler effect. The Doppler effect describes the change in the frequency of a received signal due to a relative velocity between a transmitter and receiver. Thus, if a WCD is mobile, and moving in relation to a base station, the frequency of signal transmitted between the WCD and the base station will vary. For example, if the WCD is moving away from the base station, a signal transmitted from the WCD to the base station would have a lower frequency, i.e. longer wavelength, than the original signal transmitted. Likewise, if the WCD is moving closer to the base station, a signal transmitted from the WCD to the base station would have a higher frequency, i.e. shorter wavelength, than the original signal transmitted. Signals transmitted from the base station to a moving WCD undergo the same frequency variations. Because WCDs are often used in vehicles, or high speed transit systems, correcting for the Doppler effect, or Doppler shift, can be an important factor in maintaining a robust and effective wireless communication system.
There is therefore a need in the art for techniques to provide improved performance of frequency tracking and time tracking loops in WCDs and base stations in a wireless communication system.